Cables for heavy-duty applications and in particular for mobile installations, such as mobile harbour cranes, ship-to-shore container cranes, ship un-loaders, spreaders, mining and tunnelling equipment, and windmill and windfarm are specifically designed to withstand harsh environment conditions and high mechanical stresses, such as tensile forces and torques. Within the present description, we will in general refer to heavy-duty cables, when referring to cables for heavy-duty applications and in particular, but not exclusively, for mobile installations.
An example of heavy-duty electric cable is provided in DE patent application No. 3934718, which describes an armoured trailing cable for shearer loaders in mines.
WO 01/78086 discloses an electric cable in particular for use in a pick-up system such as a crane or shelving system. The cable comprises a core, which includes first conductors, completely surrounded by and embedded within a first stress-bearing matrix. At least one further layer is disposed about the first stress-bearing matrix and has at least one further conductor in the further layer which is completely surrounded by and embedded within a second stress-bearing matrix. The stress-bearing matrices in the cable are said to allow the distribution of stress throughout the cable and thus to substantially reduce the corkscrew effect.
U.S. Pat. No. 6,247,359 describes an apparatus for identifying the need to replace a synthetic fiber rope constructed of at least two concentric layers of strands laid together and made from load-bearing aramid fiber strands comprising an indicating device visible on an exterior surface of the rope for detecting and visually indicating a rotational position of the rope about its longitudinal axis.
The Applicant has observed that conventional methods of evaluation of the torsion of a cable based on visual detection of a coloured line or marks along the cable length are often not reliable since they strongly depend on the condition of the cable external surface, for example they are affected by the presence of dirt or scratches. Furthermore, such methods generally do not provide quantitative data on the amount of torsion applied to the cable.
Non-contact torque sensors find wide application in measuring stresses in a shaft or driveline component of a vehicle during operation.
US 2007/0241890 describes an apparatus for measuring at least one physical characteristic, e.g., torque, of a shaft or driveline component of a vehicle. A radio frequency (RF) tag is associated with the shaft to facilitate communication to an RF reader. The RF tag is capable of storing a physical characteristic of the driveline component such as torque. The RF reader includes a transmitter provided to send modulated radio frequency transmissions that both supply power to the RF tag and associated sensor and trigger a responsive transmission signal indicative of sensed torque. The frequency tag reader is positioned adjacent to the driveline component and operable to read the signal transmitted by the RF tag. The RF tag may be continuously triggered and read by the RF modulator/reader in rapid cycles to facilitate continuous monitoring of object to be sensed.
Radio frequency identification (“RFID”) elements embedded in a cable can facilitate locating and identifying the cable. The RFID elements or transponders can provide information about the cable—for example, identification number, time of deployment, manufacturing batch—to a remote RFID reader without directly accessing or handling the cable. This can be particularly useful in situations such as when the cable buried underground, suspended overhead, or installed in a cable tray.
US 2007/0120684 discloses a cable identifying system used with RFID built-in cable including therein RFID tags, each RFID tag having a responder comprising a radio transmitter/receiver and a memory device, operable without physical contact, the system comprising an external information storage apparatus that is to store the entire information on the ID data stored in the memory devices incorporated in all the RFID tags included in the RFID built-in cable. An antenna used for the RFID reader with a pair of semi-cylindrical members is disclosed. It is said that by using this type of antenna, the exact location of each RFID tag cannot be detected, but the information from the antenna incorporated in the RFID tag can be effectively detected irrespective of the position of the tag in the cable.
US 2008/0204235 discloses an optical fibre cable comprising a nonconductive tape extending a length of the cable and a plurality of RFID transponders disposed periodically along a length of the tape, wherein the radio frequency identification transponders report information that can facilitate locating and identifying the cable. Each RFID element has a unique code, thereby providing a record of manufacturing parameters that are specific of that cable. The unique code can be specific to an incremental length of the cable.
In some applications, such as in heavy-duty applications, transfer of the cable to the equipment reels and forced guidance during the winding and unwinding phases may give rise to undesired torsions that can vary along the cable length. Although care is normally recommended in handling and installation of the cable in the mobile equipments, such as a direct transfer of the cable from the original drum to the cable reel while avoiding changes of direction or inversions of the original direction of winding, working conditions may induce relatively large and abrupt torques thereof In addition, other systems for cable movement, such as guidance devices, pulley systems and tender systems, may involve torsions of the cable during operation, in particular if applications require high-speed operation and/or multiple cable deflection in the cable payout.
The Applicant has tackled the problem of detecting the presence of torsion in a cable in use and of providing a reliable measurement of the actual deployment of the cable, which can be performed throughout the lifetime of the cable.